1. Field of the Invention
This invention relates to a nonvolatile semiconductor memory device and a method of manufacturing the same. More particularly, this invention relates to a nonvolatile semiconductor memory device which improves the tunnel insulating film and inter-electrode insulating film in a stacked gate configuration where a floating gate electrode and a control gate electrode are stacked one on top of the other and to a method of manufacturing the nonvolatile semiconductor memory device.
2. Description of the Related Art
A MOS structure with a stacked gate configuration has been used for the memory cells in a NAND nonvolatile semiconductor memory device. The MOS structure with a stacked gate configuration is such that a floating gate electrode is formed above a semiconductor substrate via a tunnel insulating film and a control gate electrode is formed above the floating gate electrode via an inter-electrode insulating film (or an interpoly insulating film). In this type of memory cell, to gain the electric capacitance ratio of the floating gate electrode to the control electrode, an SiO2/SiN/SiO2 film (hereinafter, referred to as an ONO film) whose permittivity is higher than that of a silicon oxide film has been used as an interpoly insulating film (refer to Jpn. Pat. Appln. KOKAI Publication No. 6-151830).
With the recent miniaturization of memory cells, the application of a material whose permittivity is higher than that of an ONO film as an interpoly insulating film has been under examination. Of the high-dielectric films, an aluminum oxide (Al2O3) film particularly has a high thermal stability and therefore is less liable to react with polysilicon. Therefore, an aluminum oxide film has good compatibility with the manufacturing process of NAND semiconductor elements and therefore is a promising film for a tunnel insulating film and an interpoly insulating film in the near future (refer to Jpn. Pat. Appln. KOKAI Publication No. 2002-539637).
However, when an aluminum oxide film is used as an interpoly insulating film, the following problem arises: when a high electric field is applied to the interpoly insulating film, the leakage level cannot be suppressed below the memory retaining characteristic.